Low alloy structural steel

ABSTRACT

Low cost, precipitation hardenable, structural steels containing carbon, nickel, molybdenum and columbium, the steels being capable of developing high strength and both ambient and low temperature toughness. Other constituents can be present, notably copper, manganese, silicon, etc.

United States Patent De Paul [4 1 Apr. 11, 1972 [54] LOW ALLOY STRUCTURAL STEEL 3,290,128 12/1966 Manganello ..7s/124 3,475,164 12/1960 l-lydrean [72] Inventor. Robert Allen De Paul, SloaLsburg, NY. 3,388,988 H1968 Nagashima U [73] Assignee: The International Nickel Company, Inc., 2,992,148 7/1961 Yeo New York, NY. 2,145,756 1/1939 Ervin 3 097 294 7/1963 Kubli 9 Y [22] Had 13 1 69 3,368,887 2/1968 Ems [21] App]. No.: 865,995

Primary Examiner-Hyland Bizot 52 US. Cl ..7s/123 J, 75/123 K, 75/124, Y Pmel 75/125 [51] Int. Cl ..C22c 39/20, C22c 39/ 14 ABSTRACT [58] held of Search 128 128 g 4 Low cost, precipitation hardenable, structural steels containing carbon, nickel, molybdenum and columbium, the steels being capable of developing high strength and both ambient [56] References cued and low temperature toughness. Other constituents can be UNITED STATES PATENTS present, notably copper, manganese, silicon, etc. 934,697 9/1909 Schneider ..75/128 V 6 Claims, No Drawings LOW ALLOY STRUCTURAL STEEL For constructional applications and the like, a steel should display considerable versatility in terms of adaptability and thus exhibit a variety of different mechanical characteristics each of which exceeds a prescribed minimum, (though each might not be necessary for all intended applications). A given steel may offer certain properties to an outstanding degree, e.g., yield strength, but be inferior as to others, e.g., toughness, the converse being equally true. Too, a particular steel may ofier acceptable toughness only in certain mill forms, e.g., bar or rod, but not in the widely used form of plate, toughness usually being much higher in the former. Plate presents an additional difficulty in the sense that in unidirectionally rolled steels, (see Metals Handbook, 8th ed. pp. 229-231) there can be a considerable disparity in toughness, depending upon whether it is measured in the longitudinal direction (direction parallel to rolling) or in the transverse direction, the lower values usually obtaining with the latter. This variance can be sufficiently great to eliminate a steel from further consideration in the absence of cross rolling.

By way of further illustration, there are steels which manifest both good strength and toughness but are otherwise unattractive by reason of the fact that they are difficulty weldable at best, weldability being a prime virtue of any steel for general purpose application. Still other steels cannot be readily fabricated into desired shapes since they do not lend themselves to such processing techniques as cold forming.

1n the light of the foregoing, the present invention is particularly addressed to the problem of providing steels which in unidirectionally rolled plate form are characterized by (a) a yield strength of about 150,000 psi or more, (b) the ability to absorb 50 ft.-lbs. of impact energy (Charpy V-Notch) in the longitudinal direction and at least 25 ft.-lbs. in the transverse direction, (c) good weldability, and (d) good formability including a reduction in area of at least about 60% Furthermore, since it would not be unexpected for a multi-purpose steel to be used in comparatively cold regions, (e) the minimum toughness values above given should obtain down to temperatures as low as minus 40 F. Moreover, the properties should be attainable without recourse to elaborate processing and by using (f) simple air melting practice together with conventional quenching and aging techniques.

Perhaps it should be mentioned that there are a limited number of weldable, formable steels commercially produced which offer both relatively good toughness and yield strengths upwards of 180,000 psi, the maraging steels being exemplary. Very few, if any, of such steels, however, are capable of delivering the toughness qualities contemplated herein and, moreover, are, comparatively speaking, more expensive. Oddly enough, a review of commercially available steels reflects that there is a considerable gap existing between the low strength 100,000 psi) and the much higher strength steels (180,000-200,000 psi and up). Neither satisfy all the objectives above discussed accordingly, there is a need for a commercial steel intermediate the low and high strength versions but which possesses those attributes so necessary for it to be useful for constructional purposes.

In any event, it has now been discovered that a low cost structural steel of novel composition and containing correlated amounts of carbon, nickel, molybdenum, columbium, manganese, silicon and other constituents as described herein can be provided which in one package satisfies each and all of the above objectives.

Generally speaking and in accordance herewith the above objectives are attained with quenched and aged steels containing, in weight percent, about 0.06% to about 0.15% carbon, about 4% to 8% nickel, from 0.6% to 1.5% molybdenum, from 0.02% to 0.15% columbium, the sum of the carbon plus columbium not exceeding 0.24%, up to about 1.75% copper, up to about 0.8% manganese, up to about 0.5% silicon, up to 0.5% chromium, up to about 0.15% aluminum, and the balance essentially iron. Elements such as phosphorus, sulfur, nitrogen, hydrogen, oxygen and the like should be kept to low levels consistent with good commercial steelmaking practice.

Phosphorus and sulfur, if any, should not exceed 0.015% each, and the total thereof should not fall above about 0.025%. Nitrogen preferably should be held to a level below 50 parts per million to minimize formation of nitrides which could otherwise induce certain unfavorable characteristics, e.g., strain aging.

In carrying the invention into practice, the carbon should not be less than 0.06%, percent, and advantageously not less than about 0.08%; otherwise, yield strength is insufficient. On the other hand, with carbon levels much above about 0.15% weldability difficulties ensue (weld and heat affected zone (HAZ" cracking) and there is an unnecessary loss in notch toughness, notably at the lower temperatures. From 0,08% to 0.13% carbon is most satisfactory.

The nickel content should not fall below 4% or 4.5%, less the danger of inadequate strength be entertained. This constituent also imparts toughness and ductility qualities and in striving for the best combination of strength and toughness it has been found that at least about 5% or 5.5% to 6.5% or 7% nickel is of considerable advantage. Amounts exceeding about (8% do not confer additional benefits of substantial consequence, particularly in view of the added cost.

As to molybdenum, it also contributes markedly to strength and toughness with the added virtue of inhibiting the occurence of temper embrittlement. However, amounts thereof much beyond 1.5% e.g., 2%, tend to promote embrittling characteristics through the formation of what is believed to be an iron molybdide compound. A range of 0.8% to 1.2% molybdenum is most beneficial.

Columbium exercises a potent influence in respect of both strength and toughness and imparts a fine grain structure. Nonetheless, relatively high amounts of columbium can bring about an impairment in impact toughness, particularly at low temperatures, a result again deemed attributable to what is believed to be an iron columbide type compound. Too, when carbon and columbium are both at the top end of their ranges weldability is endangered and thus the total percentage of these constituents should not exceed 0.23% or 0.24%. Advantageously, the columbium content is maintained between about 0.05% to 0.11%. Vanadium can be used in lieu of columbium, but the latter is much preferred since it affords a more refined grain structure, a grain size of ASTM 9 or 10 or finer, than vanadium. This is beneficial to weldability.

Copper has been found to exercise a relatively potent influence in achieving good yield strength levels in heavy section sizes, particularly in respect of sections 3 inches or more in thickness, an effect due to precipitation hardening upon cooling from aging. For sections up to 3 inches, copper is not required, but about 0.5% to 1.5%, preferably 0.75% to 1.25%, is of considerable merit for 3 inch sections and above.

With regard to other constituents, manganese, although not of an absolute necessity, should be present in an amount of at least about 0.2%, advantageously at least 0.4%, to tie up sulfur and thus minimize hot workability difficulties. It should not exceed 0.8%, and preferably is not greater than 0.6% or 0.7%, to avoid weldability problems. Again, while not absolutely essential, silicon, e.g., upwards of 0.02%, does contribute to good deoxidation and is useful in killing the steel, if desired; however, percentages above about 0.5% detract from toughness and can cause or promote poor weldability characteristics. From 0.1% or 0.15% to 0.35% is satisfactory. Aluminum also is useful for deoxidizing purposes but significant amounts thereof can impair toughness and thus are quite unnecessary. Elements such as chromium, boron and cobalt, can be tolerated in small amounts. However, significant amounts of chromium have been found to subvert toughness; thus, whether added by way of scrap or otherwise, it should be controlled so as not to exceed 0.5%, e.g., 0.25% Moreover, in the presence of carbon at the high end of the carbon range, about 0.12% up to 0.15%, the chromium content should not exceed 0,2% in order to minimize weldability difficulties.

Conventional steelmaking practice, including both air melting or vacuum processing, can be used in producing the subject steels. Vacuum melting does reduce the level of deoxidants needed which in turn yields a cleaner steel, although cost might be a factor. Standard cast iron ingot molds can be used for casting or pouring. lngots should be soaked at about melted steel compositions, Steels I through 8 being within the invention whereas A through H are outside the scope thereof.

All steels shown in Table l were prepared using an air induction furnace and magnesia crucible. Generally, 30-lb. ingots 2,000 F. to 2,300" F., a preferred range being from 2,1 75 F. were prepared, the pouring temperature being about 2,750 F. to 2,225 F.) for 2 to 4 hours depending upon the size of the Thereafter, the specimens were soaked for 2 hours at about ingot. Forging to the desired billet or slab size is accomplished" 2,200 F., forged to l-inch plate, then cooled, reheated to directly from the soaking temperature. Melts can be air cooled about 1,800 F. for about 1 hour, rolled to k -inch plate (4 to room temperature and subsequently reheated to the rolling inches wide, 14 inches long) in two passes, air cooled, solution temperature or equalized in a furnace following forging at the treated at about 1,600" F. for about 1 hour, water quenched, rolling temperature. With respect to hot working, rolling h n ge a ab u 95 F f r 3 hours and then air cooled to should be initiated over the temperature range of about l,550 room temperature. All steels were unidirectionally rolled. F. to 1,900 F., with a range of 1,775" F. to l,825 F. being Standard tensile blanks were cut from the longitudinal quite satisfactory. Rolling to the desired size and shape should ire i n n an ard Charpy w r C t from bo h he TABLE II CVN Impact Toughness Longitudinal Transverse 'Y.S U.T.S., El0ng., k.s.i. k.s.l. percent percent R.'I 40 F. R.T. 40 F be accomplished over a finishing temperature range of l,350 longitudinal and transverse directions. The results (average F. to 1,850 F., e.g., 1,600 F. to 1,750" F. Below l,700 F. is values) are reported in Table ll. preferred. Attractive properties can be achieved with finishing The above data reflect that steels within the invention are temperatures as low as l,350 F., which falls into the ferrite characterized by an excellent combination of strength and plus austenite region. The material can be air cooled or water toughness coupled with good ductility and reduction of area cooled and advantageously is sprayed or quenched following values. In marked contrast, Alloys A through H all failed in rolling. one or more respects. Alloy A, low in carbon, was notable for If a solution treatment is employed, it should be conducted its lack of strength (also true for molybdenum deficient Alloys over the temperature range of about 1,500 F. to 2,000 F. for E and F) whereas excessive carbon Alloys B and C (also high a period of about 95 hour to 3 hours, 1 hour being preferred. If molybdenum Alloy G) exhibited inadequate toughness. As the steels are water quenched relatively quickly after final hot will be detailed hereinafter, Alloys B, D and H cracked upon rolling, the steels can be directly aged for maximum yield 45 welding. strength without the need for a solution treatment. Upon A careful perusal of the data in Tables I and ll further conquenching, the steels are then aged within the range of about firm the considerable difference in toughness, i.e., the ability 800 F. to 1,l F. for i hour to 12 hours. The solution and to absorb impact energy, which can be expected when a comaging treatment temperatures are advantageously carried out parison is made between longitudinal and transverse measureover the temperatures of 1,550" F. to l,650 F. (1 hour) and 50 moms. As previously indicated herein, this is rather common 900 F. to l,000 F. (2 to 4 hours), respectively. Cold forming, metallurgical behavior with unidirectionally rolled steels. This where desired, should be effected prior to aging. disparity, however, can be greatly narrowed by cross rolling as For the purpose of giving those skilled in the art a better apevident from the data set forth in Tables ill and IV below, the preciation of the advantages of the invention, there is given steels having been processed much in the same manner as the herein data illustrative of the properties characteristic of the steels in connection with Table l except that cross rolling was subject steels. In Table i there is presented a series of air applied.

TABLE I C, Ni, Mo, Ch, 11, Si, Al. Fe, Steel percent percent percent percent percent percent percent percent 0.14 6.05 0.98 0.09 0. 53 0. 29 0. 04 1151. 0. 09 6. 00 0. 98 0.08 0. 54 0. 28 0. 04 B51. 011 9. 6 1. 01 0.11 0. 56 0. 29 0.13 Bal. 0.10 6. 0 0.98 0. 05 0. 55 0. 28 0.04 Bal. 0.11 5. 0 0. 30 0.10 0. 64 0.15 0.09 B51. 0.12 6. 2 0.95 0.10 0. 0.17 0.10 Ba]. 0.10 5. 0. 03 0.10 0. 47 0. 23 0. 04 Hal. 0. 08 5.45 0. 94 0.08 0.53 0. 28 0. 05 B51. 0.05 6. 0 0.98 0. 09 0. 55 0.31 0.05 B51. 0.16 6. 50 1. 08 0.13 0.58 0.30 0. 06 Ba]. 0. 23 6.05 0. 98 0. 00 0. 53 0.31 0. 05 Ba]. 0.11 6. 03 0. 0.15 0. 53 0.36 0. 05 Bal. 0.11 5.93 n.a. 0. 00 0. 56 0.33 0.04 B51. 0.12 6. 0 0. 40 0. 00 0. 56 0. 31 0. 05 Bal. 0.11 6.0 1. 04 0.10 0. 50 0. 28 0. 05 1331. 0.10 6. 0 0.98 0.10 0. 57 0.32 0. 05 Bal.

Contained 0.49% Cu.

2 Contained 1.03% Cu.

30011). heat (poured at 2,850 F.).

n.a.=not added.

Bal.=balance iron and impurities; all steels 0.01% of sulfur except Alloy H which contained contained less than 0.02% phosphorus and 0.024% and 0.012% of P and S, respectively.

TABLE III Ni. Mo. Cb, Mn, Si, Al, Fe, percent percent percent percent percent percent percent 6. 0 6. 07 0 95 0. 06 0. 57 0. 22 0. 076 Hal. 6. 07 0. 93 0. 19 0. 59 0. 28 0. 076 Bal 6.10 1. 00 0. 25 0. 58 0. 28 0. 077 Bnl.

TABLE IV CVN Impact Toughness Longitudinal Transverse Y.S. U.T.S. Elong., .A., Steel k.s.i. k.s.i. percent percent R.T. -40 F. R.T. -40 F.

The minimum toughness level of 50 foot-pounds required Although the present invention has been described in conherein in connection with unidirectionally rolled alloys need junction with preferred embodimentsit is to be understood not obtain for cross rolled steels. In the latter case, the steels h ifi i n Variations may be r r ed to Without should nonetheless exhibit a minimum toughness of footdeparting from the spirit and scope of the invention. as those pounds, preferably at least foot-pounds. skilled in the art will readily understand. Such modifications Since, as noted herein, alloys to be used for constructional and Variations are considered to be the purview and type applications should manifest good weldability charac- 25 1 p? fi invention and appended dams teristics, various compositions were studied in respect of c susceptibility to cracking. In this regard, bead on plate tests 363 19 gii srgiz tgi figfiggg 533 31 2 were conducted using plates 3 inches in width, 6 inches in Ion g and at least 25 ft Jbs in trallsvem length and One-half inch thick the Plates having been solution dire t ztion at tern eratures down to at least F and 00d treated for 1 hour at 1,600 F., water quenched, aged 3 hours 30 Id d 18 d f g at 950 F. and then air cooled. The faces of the plates were we a an oma Sal stee mg evol o vana um and consisting of from about 0.06% to about 0.13% car- Surface ground and beads were depmed along 'f bon, about 5% to 8% nickel, from 0.6% to 1.0% molybdenum, length of the plate, one at 16 inches per minute, representative from 002% to about 011% columbium, the sum f carbon of automatic weldmg Speeds, and one at 60 inches P minute plus columbium not exceeding about 0.23%, up to about as a thermal shock test. Each deposit was made at 15 volts and 35 1.75% percent copper, up to about 0.8% manganese up to 250 P Observation cl'ackmg was made at a magmfica' 0.5% silicon, up to 0.5% chromium, the chromium not exiion of The compositions and cracking behavior are ceeding about 0.2% when the carbon content is 0.12% or noted Table more, up to 0.15% aluminum and the balance essentlally lron.

TABLE V Behavior C, Ni, Mo, Cb, Weld cracking head at percent percent percent percent Other 1 bead at 16 i.p.m. 60 i.p.m.

0.14 6.05 0. 98 0. 09 None None 0.09 6.00 0.98 0.08 d 0. 10 6.0 0.98 0.05 0.16 6. 1. 08 0.13 0. 23 6.05 0.98 0.09 0. 11 6.03 0. 99 0.15 0.10 6.0 0.98 0. 10 0.15 6. 06 0.95 0.11 2 small cracks. 0.10 6.04 0.96 0.10 None 1 Balance of steels iron and impurities with manganese, silicon and aluminum contents being within invention.

As will be seen from the data given in Table V, than while 2. A steel in accordance with claim 1 containing from 0.8% steels within the invention failed to exhibit defects, this was to 1.2% molybdenum. not the case concerning steels beyond the invention. Alloys B 3. A steel in accordance with claim 1 in which chromium, if and C reflect that high carbon contributes to cracking, a result ny, does not exceed pe cent which also follows when the combined percentage of carbon 4- A Steel i accordance Wi h Cl m 1 which in a cti n Size plus columbium (Alloy D) is too high. Alloy H serves to inches mQre Comains I 109 emphasize that phosphorus and lf Should be maintained at 60 5. A steel in accordance with claim 1 containing from 0.2% low levels, i.e., at least below 0.02% each. Alloys J and 13 are g? g i M u m f t] t b I included to illustrate that chromium can bring about cracking 6 fi zzii rgfiz i izi g i 3: a particularly at high carbon levels Accordingly if chromium lon 'tudix ial directign and at least 25 ft lbs in the transverse must be present then it should not exceed 0.2% when the cardiregtion at temperatures down to at le'ast F and g bon from 012% Automanc MIG and manual weldability and formability, said steel consisting of from about covered electrode welding technlques have also been success- 0 06% to about 0 13% carbon about 5% to 8% nickel from fully earned out on Steels m .accorqance herewith about 0.8% to about 1.2% molybdenum, metal from the group The steels of the preserit mvennon are pamcularly apphca' consisting of columbium and vanadium in an amount from ble for structural application, e.g., pressure vessels, high pressure tubes, cold forged products, high strength fabricated structures of various shapes, etc., although they can be used, of course, wherever the above described combination of properties is deemed beneficial. They can be produced in the usual mill forms including not only plate but bar, rod, forgings, 75 etc.

bium not exceeding about 0.23%, up to about 1.75% copper, up to about 0.8% manganese, up to 0.5% silicon, up to 0.25% chromium, the chromium not exceeding about 0.2% when the carbon content is 0.12% or more, up to 0.15% aluminum and the balance essentially iron.

about 0.05% to about 0.1 l the sum of the carbon plus colum- 

2. A steel in accordance with claim 1 containing from 0.8% to 1.2% molybdenum.
 3. A steel in accordance with claim 1 in which chromium, if any, does not exceed 0.25 percent.
 4. A steel in accordance with claim 1 which in a section size of 3 inches or more contains at least 0.5% copper.
 5. A steel in accordance with claim 1 containing from 0.2% to 0.7% manganese.
 6. An alloy steel possessing a yield strength of at least about 150,000 psi, an impact toughness of at least 50 ft.-lbs. in the longitudinal direction and at least 25 ft.-lbs. in the transverse direction at temperatures down to at least -40* F., and good weldability and formability, said steel consisting of from about 0.06% to about 0.13% carbon, about 5% to 8% nickel, from about 0.8% to about 1.2% molybdenum, metal from the group consisting of columbium and vanadium in an amount from about 0.05% to about 0.11 the sum of the carbon plus columbium not exceeding about 0.23%, Up to about 1.75% copper, up to about 0.8% manganese, up to 0.5% silicon, up to 0.25% chromium, the chromium not exceeding about 0.2% when the carbon content is 0.12% or more, up to 0.15% aluminum and the balance essentially iron. 